For intercommunication and interaction between the viscera two systems were elaborated: a younger system of direct contacts, the nerves, and nerve cells, through which influences could be conducted for the stimulation, acceleration, retardation or inhibition of an energy process in them; and the older, the endocrine gland association, for the production of chemical substances to act as messengers to be sent from one viscus to another, and also to the nerves, through the blood or lymph which bathe all the cells. They could affect only one or certain organs, because by selection only the chosen organ or organs knew the code, as it were. The chemical system is much the older system, and preceded the nerve system by aeons of time. The whole system, viscera, visceral nerves and the endocrines gradually united into a complete autonomous organism within the organism, and as such functions as the vegetative apparatus. EVOLUTION OF THE ENDOCRINES 2. In the course of evolution, variations occurred in all three components of the apparatus, the viscera, the nerves, and the endocrines. Now variations in the viscera and the nerves are essentially grossly physical and quantitative. That is, there may be a bigger stomach or a smaller stomach, larger nerve fibres or smaller. And as Life always has worked with a large margin of safety, and always played for safety first as regards quantity, these variations have not become of much significance for the history and destiny of the animal. But variations among the endocrines made a tremendous difference. To have very much thyroid and very little pituitary, much adrenal and not enough parathyroid meant a great deal to the Organism as a whole, as well as to the vegetative apparatus. For states of tension and relaxation, activity and inactivity in the nerves and viscera would be determined by these variations in the ratio between the variants. The vegetative apparatus in its virginity, say in the new-born infant, may be said to have its development primarily determined by the reaction potentials of the endocrine part of it, that is the latent power of each gland to secrete at a minimum or a maximum, and the balance between them. EDUCATION OF THE VEGETATIVE SYSTEM 3. Training or education involves, beside other effects, a training of the endocrines, and hence of the entire vegetative apparatus, to respond in a particular way to a particular stimulus. Experience is like the introduction of new push-buttons, levers, and wheels into the mechanism. All learning which calls out or arrests the functioning of an instinct, must, from what we have learned of the chemical dynamics of instincts as reactions between hormones, nerves and viscera, affect the vegetative system. When there is a conflict between two or more instincts, between pressures of energy flowing in different directions, there may be compromise and normality, or a grinding of the gears and abnormality.